The neuropathological hallmarks of Alzheimer's disease (AD) consist of extracellular amyloid plaques, amyloid angiopathy, neurofibrillary tangles, inflammation, dystrophic neurites, and neuronal death. Rare, autosomal dominant familial forms of AD (FAD) have provided us with most of what is currently known of the molecular basis of the disease. Most FAD cases result in mutations in a polytopic membrane protein, presenilin 1 (PS1), that contains the active site for the 3-secretase enzyme, and which, in turn, determines the carboxyl terminus of the amyloid beta peptide (A2). Most A2 peptides end at residue 40, but a minor quantity ends at residue 42 and are highly prone to aggregation. The amyloid hypothesis of AD suggests that this increase in A2-42/40 results in a cascade that ultimately leads to hyperphosphorylated tau, synaptic dysfunction, and neuronal death. When new FAD families are identified, routinely, the PS1 gene is sequenced, and if a PS1 mutation is the cause, then either missense or deletion mutations will be found in the DNA of affected but not unaffected family members. Such was the case of the Tasmanian (Tas-1) family. Kwok et al. (2003) described the pathology and the genetics of the Tas-1 family; the pathogenic mutation in Tas-1 is a missense mutation, L271V, which disrupts alternative splicing of the PS1 gene, and results in a gene that lacks exon 8 (PS1 8). The goal of the current project is to characterize this novel FAD pathogenic PS1 mutation. Based on the work of Kwok et al. (2003), the overall objectives and methods of the project will be to determine if PS1 8 is catalytically inactive. Preliminary data showed that crossing the PS1 8 mutation with a PS1 (-/-) knock-out (KO) mouse did not rescue the KO phenotype seen in PS1 (-/-) KO animals, as they are embryonic lethal. In the current proposal, behavioral deficits and levels of A2-42, A2-42/40, and A2 oligomers will be determined in PS1 8 mice crossed with a mouse model harboring a mutation in APP. Also, histological analyses will be utilized and APP cleavage will be determined in these mouse models. Finally, further to evaluate whether PS1 8 is a loss of function mutation and to distinguish between the older dimer model and the more recent 1:1:1:1 model of the 3-secretase complex, cell culture, co-immunoprecipitation, confocal microscopy, and alpha screen techniques will be employed to evaluate whether PS1 8 is interacting with wt PS1 resulting in 3-secretase's altered substrate cleavage. Resolving the function of this protein in vivo and in vitro will allow us to further understand the mechanisms underlying FAD and possibly elucidate PS1 as a putative target of therapeutic intervention for AD.